Structure Determination of Enalapril Maleate Form II from High

Structure Determination of Enalapril Maleate Form II from HighResolution X-ray Powder Diffraction Data
Y.-H. Kiang*†, Ashfia Huq‡, Peter W. Stephens‡, Wei Xu†
† Pharmaceutical Research and Development, Merck Research Laboratories,
Merck & Co., Inc., P.O. Box 4, West Point, PA, 19426
‡ Department of Physics and Astronomy, State University of New York,
Stony Brook, NY, 11794
Accepted
Journal of Pharmaceutical Sciences
March 5, 2003
*Author to whom correspondence should be addressed. ([email protected])
Abstract
The crystal structure of polymorphic form II of enalapril maleate, a potent angiotensin
converting enzyme (ACE) inhibitor, was determined from high-resolution X-ray
diffraction data using the direct space method. Enalapril maleate Form II crystallizes in
space group P212121, Z=4, with unit cell parameters a=33.9898(3), b=11.2109,
c=6.64195(7) Å, and V=2530.96(5) Å3. By treating the molecules as rigid bodies and
using the bond lengths and angles obtained from the X-ray single crystal structures of
Form I, which were solved almost 20 years ago, the total degrees of freedom of enalapril
maleate were reduced from 25 to 12. This reduction in total degrees of freedom allowed
the simulated annealing to complete within a reasonable computation time. In the crystal
structure of Form II, the crystal packing, hydrogen-bonding pattern, and conformation of
enalapril maleate resemble those in the structure of Form I. The crystal packing and
conformation of enalapril maleate in the two polymorphic forms may explain the
similarity of thethermal properties,
13
C-NMR, FT-IR, and Raman spectra of Form I and
II. In both structures, the conformations of the main peptide chains, which are considered
responsible for binding the active ACE sites, remain largely unchanged. Lattice energy
calculation showed that Form II is slightly more stable than Form I by 3.5 kcal/mole.
Keywords: enalapril maleate, polymorphism, crystal structure determination, X-ray
powder diffraction (XRPD) pattern, Monte-Carlo/Simulated Annealing
Introduction
2
Since the development of captopril and enalapril, 1-2 angiotensin converting
enzyme (ACE) inhibitors have attracted much attention for their applications in control of
hypertension. X-ray single crystal structures of many ACE inhibitors have been
determined and the conformations analyzed in attempt to use the results as an aid in the
design of new ACE inhibitors.3-12 The X-ray single crystal structure of enalapril maleate
Form I was solved and reported by two independent groups in 1986.3-4 The reported
structures were since used in a number of research papers to provide conformational
information of the enalapril molecule in solid state.3-4,12-13 Enalapril maleate, however,
exists in another polymorphic form denoted as Form II,14 the structure of which has not
been reported to date.
C2H5O2C
N
H
CO2H
N
O
•
CO2H
CO2H
Polymorphism, the existence of two or more crystal forms of the same compound,
is very common in pharmaceutical solids.15-16 Characterization and control of
polymorphism has been of particular importance to the pharmaceutical industry as
change of polymorphs can alter the bulk properties, bioavailability, and the chemical and
physical stability of a drug. The fundamental understanding of polymorphism comes
from the knowledge of crystal structure at the atomic level. However, determination of
crystal structure is often challenging due to the difficulty of growing single crystals
suitable for X-ray analysis. Polymorphs are therefore routinely identified using X-ray
powder diffraction patterns and the structural information is obtained indirectly from
XRPD, solid state NMR, IR and Raman. In the case of enalapril maleate, two
polymorphic forms were reported in 1986 by Ip and co-workers using spectroscopic data
and solution calorimetry. 14 The XRPD patterns and solid state
13
C NMR spectra of these
two polymorphs are shown in Figure 1 and 2, respectively. Based on the spectroscopic
data, the two polymorphs of enalapril maleate are concluded to be very similar in their
structures. However, the X-ray single crystal structure of the Form II has never been
obtained and probably will never be as the Form II is made from water slurry of Form I.
3
In the aforementioned research papers of ACE inhibitors, the conformational information
of enalapril was taken from Form I, the only known crystal structure. A crystal structure
solution of the Form II will therefore provide clearer understanding of the polymorphism
of enalapril maleate as well as more complete information on the enalapril conformation
in the solid state.
Crystal structure determination based on XRPD patterns using the direct space
method has become increasingly important for pharmaceutical solids with the increasing
use of high resolution x-ray diffraction data, the development of new computer programs,
and improved search algorithms.17-19 In the past 5 years, more than 10 crystal structures
of pharmaceutical solids have been determined based on X-ray powder diffraction
patterns.20-24 Among the currently available search algorithms for direct space method,
Monte-Carlo/Simulated Annealing (MC/SA) has been most widely used.25-28 This search
algorithm employs random sampling coupled with simulated temperature annealing in
order to locate the global minimum of the figure-of-merit factor. As the computing time
increases exponentially with the increase of total degrees of freedom, size of the system
is critical to the success of the MC/SA search. When MC/SA is used in the direct space
method for solving crystal structures from powder diffraction data, large flexible
molecules or salts with counter ions are usually considered non-favorable because of the
higher total degrees of freedom. Nonetheless, prior experimental knowledge on the
molecules from single crystal structures of other polymorphs sometimes may be useful to
reduce the total degrees of freedom. For a complete structure determination, the structural
solution obtained form MC/SA should be subsequently subject to refinement for the
atomic positions and thermal factors.
In this work we report the crystal structure of enalapril maleate Form II from
high-resolution synchrotron X-ray powder diffraction data. The structural solution was
obtained using a MC/SA search algorithm and the atom positions were refined
individually. The crystal packing of Form II and I was analyzed and the analysis was
used to understand the solid state
13
C NMR and IR spectra of the two forms. The
molecular conformation of enalapril in Form II was compared to the one in the Form I
4
crystal structure. The crystal structures were also used to calculate the lattice energies of
the two polymorphic forms.
Experimental Section
General Procedure: The polymorphic Form II of enalapril maleate ((S)-1-[N-[1(ethoxycarbonyl)-3-phenylpropyl-L-analyl]-L-proline maleate) was obtained from Merck
& Co., Inc. (>98% purity), and used without further recrystallization. Slow evaporation
of the methanol solution of enalapril maleate yielded Form I. The samples were sealed in
0.7mm special glass capillary tubes, and powder X-ray diffraction data were collected on
an Inel MPD X-ray diffractometer equipped with a CPS 120 detector at 35 kV, 30 mA,
for Cu Kα1 (λ =1.5406Å) monochromated by a Germanium(111) crystal. A mixture of
silicon and silver behenate was used as an external standard.
Synchrotron X-ray diffraction measurement was performed on beam-line X3B1 at
the National Synchrotron Light Source, Brookhaven National Laboratory. The X-ray
wavelength of 1.1508Å was selected by double crystal Si(111) monochromator. The
diffracted beam was selected using a Ge(111) analyzer and detected with a Na(Tl)I
scintillation counter with a pulse-height discriminator in the counting chain. The
diffracted intensity was normalized to the incident beam, monitored by an ion chamber.
The powder sample was sealed in a 1.5 mm thin-wall glass capillary tube and X-ray
diffraction data were recorded with a step size of 0.005º and a counting time 2 sec/step
from 2º to 20º in 2θ, increasing quadratically to 17 sec/step at 47.65º. During data
collection the sample was rotated to reduce the effect of sample granularity. The peaks
were fitted by a local deconvolution program and powder data were indexed by the
computer program ITO29 into an orthorhombic cell, giving a figure of merit M(20) of
308. The systematic absences suggested P212121 as a possible space group.
Infrared spectra were recorded on a Perkin Elmer FT-IR spectrometer Spectrum
One using reflective mode: number of scans was 64; resolution was 1 cm-1 ; range was
4000-400 cm-1. High-resolution
13
C spectra were obtained by cross-polarization magic-
angle spinning (CPMAS) experiments at 100.627 MHz on a Bruker DSX-400 WB NMR
5
spectrometer with a 7 mm H/X CPMAS probe. Approximately 200 mg of sample was
placed in a Zirconia rotor sealed with Kel-F caps under ambient conditions and spun at 7
kHz during the experiment. Cross-polarization was done at Hartman-Hann condition.
The contact time and the repetition delay were optimized to 2 ms and 5 s, respectively.
The free induction decay was acquired for 50 ms; 4 K data points were collected and
zero-filled to 8 K before transformation using 20 Hz of line broadening. For each form
600 scans were accumulated. Glycine resonance at 43.6 ppm was used as an external
standard for chemical shift assignments referenced to TMS.
Structure Solution: The structure determination from the synchrotron X-ray powder
diffraction pattern was carried out using the MC/SA program PowderSolve, which is
incorporated in the molecular simulation package Material Studio.30 The molecular
models of the enalapril and maleate were obtained from the single crystal structure of the
Form I and used without further optimization. For the MC/SA search algorithm, the total
degrees of freedom of the system is important to the success of the simulation. In the
enalapril maleate system, the total degrees of freedom for enalapril is 17 (11 torsional, 3
translational, and 3 rotational), and for maleate is 8 (2 torsional, 3 translational, and 3
rotational). Although at the high end, the total 25 degrees of freedom were within the
computation limit of current computers and MC/SA programs. However, trials with this
many degrees of freedom using the two programs PowderSolve and PSSP19 were not
successful. Since the Form I single crystal structure was known and considered
structurally similar to the Form II based on the XRD patterns and the
13
C NMR spectra,
we tried to search a restricted space. Rigid molecular fragments of enalapril and maleate
from the single crystal structure of Form I were used to start the MC/SA with all torsional
degrees of freedom fixed. This approach reduced the total degrees of freedom from 25 to
12 and made the MC/SA search process for the Form II crystal structure much more
rapid. The final Rwp (Rwp=[∑iwi|Iexp(θi)-Icalc(θi)|2 / ∑iwi| Iexp(θi)|]1/2) from the first three
cycles of MC/SA was 17.51%. The torsion angles of the enalapril molecule were then
released to refine while the torsional degrees of freedom for the maleate molecules as
well as the intermolecular degrees of freedom were fixed for the subsequent MC/SA
6
search. Three cycles of MC/SA search further reduced the Rwp factor from 17.51% to
14.04%. An iterative process of selectively refining part of the total degrees of freedom
resulted in a convergent final Rwp factor of 13.66%, indicating the structural model was
successfully located by the MC/SA search. This process is summarized in Table 1. The
structural solution obtained from PowderSolve was subsequently used for Rietveld profile
refinement with the program GSAS.31
In the Rietveld refinement the atomic positions of all non-hydrogen atoms were
refined without any constraints. The overall temperature factors were refined to two
numbers: one for the enalapril molecule and the other for the maleate molecule. The final
Rwp of the Rietveld refinement was 7.61%, indicating the reliability of the refinement.
The experimental XRPD pattern, the powder pattern calculated for the fully refined
structure, and the difference between the two patterns are shown in Figure 3. Table 2 lists
the Rietveld refinement information and the crystal data of enalapril maleate Form II. The
higher value of χ2, 31.1, is due to the high counting statistics and the excellent signal-tobackground ratio of the raw experiment data. The hydrogen atoms were included after the
refinement at their geometrically constrained positions. The atomic coordinates and
isotropic displacement parameters are listed in Table 3.
Results and Discussion
The crystal structure of enalapril maleate Form I is illustrated in Figure 4. In this
structure there is only one crystallographically non-equivalent enalapril maleate
molecule. After the symmetry operations of the space group, four enalapril maleate
molecules were generated in the unit cell. The maleate molecule is found considerably
deviated from planarity. This deviation is consistent with the maleate in the Form I single
crystal structure and has to be attributed to hydrogen bonding and crystal packing effects.
The single crystal structure of Form I indicated that the enalapril maleate molecule is
protonated at the analyl N and this information is used in the Form II structure. Based on
the distance between the alanyl nitrogen atom and one of the maleate oxygen atoms
(2.76Å), the enalapril molecule is hydrogen bonded to the maleate through the protonated
7
alanyl nitrogen atom. An additional hydrogen bond links the analyl N atom to the
carboxylate oxygen atom of an adjacent enalapril. These hydrogen bonded enalapril
molecules are symmetrically related by a 2-fold screw axis along the c axis. Apart from
these hydrogen bonds, all other intermolecular contacts correspond to normal van der
Waals interactions.
The crystal packing of the two polymorphs is shown in Figure 5. In Figure 5, one
sees that the two polymorphic forms of enalapril maleate are of very similar crystal
packing pattern as expected from the spectroscopic data. The space group of Form I is
P21. In P21 the only symmetry operation is a 2-fold axis along the b axis, which is
pointing into the paper in Figure 5. All the enalapril maleate molecules in Form I are
related to one another by this 2-fold screw axis. On the other hand, Form II is crystallized
in P212121, in which three 2-fold screw axes are present along a, b, and c directions. In
both figure 5a and b, the eight enalapril maleate molecules can be divided into two
groups each containing four molecules, the first group being at the top and the second
group at the bottom of the figure. In the structure of Form II, within each group the four
enalapril maleate molecules are related by a 2-fold screw axis along the c axis, which is
pointing into the paper as the b axis in Form I. However, unlike the Form I structure, the
two groups in Form II are related by a 2-fold screw axis along the b axis, which in Figure
5b is lying on the paper horizontally from left to right. The analyl carbonyl groups in
Form I are all pointing into the paper; however, in the structure of Form II the analyl
carbonyl groups in the first group are pointing out of the paper while in the second group
they are pointing into the paper.
The main difference in crystal packing between the two polymorphs lies in the
distance of the two phenyl rings of adjacent enalapril molecules. In Form I, the closest
distance between the two phenyl rings is about 3.5Å and the farthest is 6.6Å, while in
Form II the closest distance is 3.8Å and the farthest is 7.5Å. In both structures, based on
the distance of the two phenyl rings, the interaction between the phenyl rings is
considered weaker than normal π−π interaction. This difference in crystal packing of the
two forms is also supported by the IR and 13C solid-state NMR spectra. The IR (Figure 6)
8
spectra of the two forms are almost identical except in the region of 750 cm-1, which is
the region of an out-of-plane phenyl ring-hydrogen interaction. In the
13
C solid state
NMR of the two forms shown in Figure 7 and 8, the main differences occur in the
aromatic ring and methyl group regions. These NMR data are in agreement with what
one sees in Figure 5, in which the phenyl ring stacking and the methyl group orientation
are the main differences in the crystal packing.
The conformation of the enalapril maleate molecule in both crystal Form I and II
along with the atom numbering are shown in Figure 9. The selected torsion angles of the
two forms are summarized in Table 4. In Table 4 the torsion angles of the peptide chains
are almost identical in the two structures. In standard nomenclature, the alanine peptide
main chain is described by torsional angles φ1, ψ1, and ω; the proline peptide chain is
described by φ2 and ψ2. In both structures, the observed rotamer around the proline is
trans (ω close to -180°). At the alanine, the three torsion angles ϕ1, ψ1, and ω correspond
to a nearly fully extended conformation. Two angles, ω and φ2, defining the spatial
orientations of amide carbonyl and carboxy groups are restricted in the antiperiplanar and
synclinal regions respectively. The selected torsion angles are listed in Table 5. It is
noteworthy to find that the only difference in the enalapril conformations of the two
polymorphs exists in the dihedral angle C15-O17-C18-C19 of the esterified carboxylate
group. This dihedral angle was considered of no significance in defining the required
conformation for ACE inhibitors. As a matter of fact, enalapril is the prodrug of its active
form, enalaprilat, in which the ethyl ester group of enalapril is hydrolyzed. In the
enzyme-ligand binding model, this hydrolyzed ester, the carboxyl group at C14, is the
actual zinc-binding group. All the anchoring sites of enalapril in the two polymorphs
therefore have almost identical torsion angles. The enalapril conformations determined in
solid-state are also in agreement with the in-solution conformation determined based on
NMR analyses.13
The similarity in the conformations of enalapril molecule in the two polymorphs
supports the conclusion by Pascard and co-workers.12
In their study, the crystal
structures of six ACE inhibitors including hydrates and solvates were analyzed and found
9
to adopt similar conformations in the peptide chains. Since the similar conformations
were found throughout different solvates and cell dimensions, Pascard concluded that the
commonly observed conformation was likely due to an energy minimum rather than
packing effect.
Finally, the crystal structures of the two polymorphic forms were used to calculate
their packing energies.32 In this calculation the force field “Dreiding 2.21” was used with
the Ewald summation for both the van der Waals and Coulombic terms.33 The force field
“Dreiding 2.21” was chosen based on reports that the Dreiding force field can model
structures with hydrogen bonds between C=O and N-H particularly well.34-36 Since
enalapril maleate is a charged salt, the Coulombic interaction is expected to contribute the
most to the total energy of the crystal. The molecular electrostatic potential of enalapril
maleate was therefore a deciding factor for the calculated energy and was obtained
according to the restricted Hartree-Fock formalism (RHF) at the 6-31G** level with the
quantum mechanic program Gaussian97.37
The calculated lattice energies (at the
temperature of 0 K) per asymmetric unit for Form I and II are -86.46 and -90.10
kcal/mole, respectively. Ip and co-workers have reported the heat of solutions for the two
forms and showed that Form II is slightly more stable than Form I by 0.6 kcal/mole.14
Our energy calculation showed that Form II is slightly more stable than Form I by 3.5
kcal/mol. This observation that Form II is slightly more stable than Form I may also be
understood by the density rule. The densities of the two forms are 1.27 (Form I) and 1.29
(Form II) g/cm3. The Kitaigorodskii closest packing model states that the basic factor that
affects free energy is the packing density.38 The denser or more closely packed crystal
has the lower free energy. The crystal structures of the two forms were both solved at
room temperature and in both structures only one crystallographically non-equivalent
molecular pair was found. As Form II is slightly denser than Form I, it is expected that
Form II is slightly more stable than Form I at room temperature based on the
Kitaigordskii density theory.
Acknowledgments
10
The authors are grateful to Dr. Robert Wenslow and Mr. Russell Ferlita of
Analytical Research, Merck & Co., Inc. for obtaining the solid state NMR data. We wish
to thank Prof. Stephen Lee of Cornell University for use of his X-ray diffractometer.
Research carried out in part at the National Synchrotron Light Source at Brookhaven
National Laboratory, which is supported by the US Department of Energy, Division of
Materials Sciences and Division of Chemical Sciences. The SUNY X3 beamline at NSLS
is supported by the Division of Basic Energy Sciences of the US Department of Energy
under Grant No. DE-FG02-86ER45231.
References:
11
1. Ondetti MA, Rubin, B, Cushman DW. 1977. Design of specific inhibitors of
angiotensin-converting enzyme: new class of orally active antihypertensive agents.
Science 196:441-444.
2. Patchett AA, Harris E, Tristram EW, Wyvratt MJ, Wu MT, Taub D, Peterson ER,
Ikeler TJ, ten Broeke J, Payne LG, Ondeyka DL, Thorsett ED, Greenlee WJ, Lohr
NS, Hoffsommer RD, Joshua H, Ruyle WV, Rothrock JW, Aster SD, Maycock AL,
Robinson FM, Hirschmann R, Sweet CS, Ulm EH, Gross DM, Vassil TC, Stone CA.
1980. A new class of angiotensin-converting enzyme inhibitors. Nature 288:280-283.
3. Fujinaga M, James MNG. 1980. SQ 14,225:1-(D-3-mercapto-2-methylpropionyl)Lproline. Acta Crystallogr B36:3196-3199.
4. Précigoux G, Geoffre S, Leroy F. 1986. N-(1-Ethoxycarbonyl-3-phenylpropyl)-Lalanyl-L-prolinium-hydrogen maleate (1/1), enalapril (MK-421). Acta Crystallogr
C42:1022-1024.
5. In Y, Shibata M, Doi M, Ishida T, Inoue M, Sasaki Y, Morimoto S. 1986.
Conformational similarities of angiotensin-converting enzyme inhibitors: X-ray
crystal structures. Chem Commun 473-474.
6. Wyvratt M, Tristram EW, Ikeler TJ, Lohr NS, Joshua H, Springer JP, Byron HA,
Patchett AA. 1984. Reductive amination of ethyl 2-oxo-4-phenylbutamoate with Lalanyl-L-proline. Synthesis of enalapril maleate. J Org Chem 49:2816-2819.
7. Paulus EF, Hennings R, Urbach H. 1987. Structure of the angiotensin-converting
enzyme inhibitor ramiprilat (HOE 498 diacid). Acta Crystallogr C43:941-5.
8. Luger P, Schnorrenberg G. 1987. A highly potent angiotension converting enzyme
inhibitor:
(S,S,S)-5-[N-(1-carboxy-3-phenylpropyl)alanyl]-4,5,6,7-
tetrahydrothieno[3,2-c]pyridine-4-carboxylic acid monohydrate, SBG 107. Acta
Crystallogr C43:484-8.
9. Attwood MR, Francis RJ. 1984. New potent inhibitors of angiotensin converting
enzyme. FERB Lett 165:201-206.
10. Attwood MR, Hassall CH, Krohn A, Lawton G, Redshaw SJ. Chem Soc, Perkin
Trans 1 1011-19.
11. Thorsett ED, Harris EE, Aster SD, Peterson ER, Synder JP, Springer JP, Hirshfield J,
Tristram EW, Patchett AA, Ulm EH, Vassil TC. 1986. Conformationally restricted
12
inhibitors of angiotensin converting enzyme: Synthesis and computations. J Med
Chem 29:251-260.
12. Pascard C, Guilhem J, Vincent M, Rémond G, Portevin B, Laubie M. 1991.
Configuration and preferential solid-state conformations of perindoprilat (S-9780).
Comparison with the crystal structures of other ACE inhibitors and conclusions
related to structure-activity relationships. J Med Chem 34:663-669.
13. Sakamoto Y, Sakamoto Y, Oonishi I, Ohmoto T. 1990. Conformation analysis of
enalapril (MK-412) in solution by 1H and 13C NMR. J Mol Struct 238:25-224.
14. Ip DP, Brenner GS, Stevenson JM, Lindenbaum S, Douglas AW, Klein SD,
McCauley JA.1986. High resolution spectroscopic evidence and solution calorimetry
studies on the polymorphs of enalapril maleate. Int J Pharm 28:83-191.
15. Byrn SR, Pfeiffer RR, Stowell JG. 1999. Solid-State Chemistry of Drugs, 2nd edition.
SSCI, Inc., West Lafayetter, Indiana, USA.
16. Haleblian J, McCrone W. 1969. Pharmaceutical applications of polymorphism. J
Pharm Sci 58:911-929.
17. David WIF, Shankland K, Shankland N. 1998. Routine determination of molecular
crystal structures from powder diffraction data. Chem Commun 931-932.
18. Engel GE, Wilke S, König O, Harris KDM, Leusen FJJ. 1999. PowderSolve- a
complete package for crystal structure solution from powder diffraction patterns. J
Appl Cryst 32:1169-1179.
19. Pagola S, Stephens PW. 2000. Towards the Solution of Organic Crystal Structures
by Powder Diffraction.
Materials Science Forum 321:40-45.
Program and
documentation available at http://powder.physics.sunysb.edu.
20. Dinnebier RE, Sieger P, Herbert N, Shankland K, David WIF. 2000. Structure
characterization of three crystalline modifications of telmisartan by single crystal and
high-resolution X-ray powder diffraction. J Pharm Sci 89:1465-1478.
21. Giovannini J, Perrin M-A, Louër D, Leveiller F. 2001. Ab initio crystal structure
determination of three pharmaceutical compounds from x-ray powder diffraction
data. Materials Science Forum 378-381:582-587.
13
22. Kariuki BM, Psallidas K, Harris KDM, Johnston RL, Lancaster RW, Staniforth SE,
Cooper SM. 1999. Structure determination of a steroid directly from powder
diffraction data. Chem Commun 1677-1678.
23. Tedesco E, Giron D, Pfeffer S. 2002. Crystal structure elucidation and morphology
study of pharmaceuticals in development. CrytEngComm 4:393-400.
24. Smith EDL, Hammond RB, Jones MJ, Roberts KJ, Mitchell JBO, Price SL, Harris
RK, Apperley DC, Cherryman JC, Docherty R. 2001. The determination of the
crystal structure of anhydrous theophylline by x-ray powder diffraction with a
systematic search algorithm, lattice energy calculations, and
13
C and
15
N solid-state
NMR: a question of polymorphism in a given unit cell. J Phys Chem B 105:58185826.
25. Andreev YG, MacGlashan GS, Bruce PG. 1997. Ab initio solution of a complex
crystal structure from powder-diffraction data using simulated-annealing method and
a high degree of molecular flexibility. Phys Rev B 55:12011-12017.
26. Putz H, Cshon JC, Jansen M. 1999. Combined method for ab initio structure solution
from powder diffraction data. J Appl Crystallogr 32:864-870.
27. Coelho AA. 2000. Whole-profile structure solution from powder diffraction data
using simulated annealing. J Appl Crystallogr 33:899-908.
28. Pagola S, Stephens PW, Bohle DS, Kosar AD, Madsen SK. 2000. The structure of
malaria pigment α-haematin. Nature 404:307-10.
29. Visser JW. 1969. A fully automatic program for finding the unit cell from powder
data. J Appl Crystallogr 2:89-95.
30. Material Studio. Accelrys Inc. 2001. San Deigo, CA, USA.
31. Larson AC, von Dreele RB. 2000. GSAS: General Structure Analysis System. Los
Alamos National Laboratory. Report LAUR 86-748.
32. Cerius2. Computer Simulation Software Package. Accelrys Inc. 2001. San Deigo, CA,
USA.
33. Mayo SL, Olafson BD, Goddard WA III. 1990. DREDING: a generic force filed for
molecular simulations. J Phys Chem 94:8897-8909.
14
34. Buttar D, Charlton MH, Docherty R, Starbuck J. 1998. Theoretical investigations of
conformational aspects of polymorphism. Part 1: o-acetamidobenzamide. J Chem
Soc, Perkin Trans 2 763-772.
35. Payne RS, Roberts RJ, Rowe RC, Docherty R. 1999. Examples of successful crystal
structure prediction: polymorphs of primidone and progesterone. Int J Pharm
177:231-245.
36. Roberts RJ, Payne RS, Rowe RC. 2000. Mechanical property predictions for
polymorphs of sulphathiazole and carbamazepine. Eur J Pharm Sci 9:277-283.
37. Frisch MJ, Trucks GW, Schlegel HB, Gill PMW, Johnson BG, Wong MW, Foresman
JB, Robb MA, Head-Gordan M, Replogle ES, Gomperts R, Andres JL, Raghavachari
K, Binkley JS, Gonzalez C, Martin RL, Fox DJ, Defrees DJ, Baker J, Stewart JJP.,
Pople JA, 1993. Gaussian 92/DFT, Revision G.1. Gaussian Inc. Pittsburgh, PA, USA.
38. Kitaigoroskii AI. 1961. Organic Chemical Crystallography. Consultant Bureau, New
York, NY, USA.
15
Figure Captions
Figure 1. XRD patterns of enalapril maleate (a) Form I and (b) Form II. Data were taken
on a laboratory diffractometer using Cu Kα1 radiation.
Figure 2. Solid state 13C NMR spectra of enalapril maleate (a) Form I and (b) Form II.
The resonances marked with asterisks are sidebands.
Figure 3. Final Rietveld plot of enalapril maleate Form II. (observed: +, calculated, -, and
difference: bottom)
Figure 4. Stereoview of crystal structure enalapril maleate Form II. Hydrogen bonding is
represented by dashed line.
Figure 5. Viewing down a 2-fold screw axis of enalapril maleate (a) Form I and (b) Form
II. Carbon atoms are shown in green, nitrogen in yellow and oxygen in red. Hydrogen
atoms are removed for clarity.
Figure 6. Infrared spectra of enalapril maleate (a) Form I and (b) Form II showing the
range 900 to 600 cm-1.
Figure 7. Solid-state
13
C NMR of enalapril maleate (a) Form I and (b) Form II showing
the range 110 to 160 ppm.
Figure 8. Solid-state
13
C NMR of enalapril maleate (a) Form I and (b) Form II showing
the range 0 to 40 ppm.
Figure 9. The conformation of enalapril in (a) Form I along with atom numbering, and in
(b) Form II.
16
Table 1. Degrees of Freedom and Rwp Factors in Each MC/SA Search Cycle
MC/SA
torsion
translation+rotation
DOF
Rwp
enalapril
maleate
enalapril
maleate
%
1
fixed
fixed
refined
refined
12
17.51
2
refined
fixed
fixed
fixed
11
14.04
3
fixed
fixed
refined
refined
12
13.69
4
fixed
refined
fixed
refined
8
13.66
17
Table 2. Crystal Data and Structure Refinement for the Form II
Empirical Formula
C24H32N2O9
Temperature (K)
297
Crystal System
Orthorhombic
Space Group
P212121
a (Å)
33.9898(3)
b (Å)
11.2109(1)
c (Å)
6.64195(3)
3
V (Å)
2530.96(5)
Z
4
3
Calculated density (g/cm )
1.29
Profile function
Pseudo-Voigt
2.0-47.65
Scan range (º2θ)
0.005
Step size (º2θ)
Counting time/step (s)
2.0-17.0
Wavelength (Å)
1.1508
Number of data points
88000
Number of parameters refined
122
Rwp
0.0761
2
31.1
χ
18
Table 3. Fractional Atomic Coordinates and Equivalent Isotropic Displacement
Parameters for the Form IIa
x
y
z
Uiso
C1
0.7161(4)
0.3362(12)
-0.0364(19) 0.0441(8)
C2
0.7017(4)
-0.2149(11) 0.0440(2)
0.0441(8)
N3
0.7183(3)
-0.2180(10) 0.2417(19)
0.0441(8)
C4
0.7449(4)
-0.3204(13) 0.2742(20)
0.0441(8)
C5
0.7519(4)
-0.3733(10) 0.0675(22)
0.0441(8)
C6
0.7800(3)
-0.2749(12) 0.3683(22)
0.0441(8)
O7
0.8002(2)
-0.1768(8)
0.3246(12)
0.0441(8)
O8
0.7982(2)
-0.3400(8)
0.5059(12)
0.0441(8)
C9
0.7032(3)
-0.1414(13) 0.3960(19)
0.0441(8)
O10 0.7243(3)
-0.1537(8)
0.5770(12)
0.0441(8)
C11 0.6696(4)
-0.0476(12) 0.3537(21)
0.0441(8)
C12 0.6984(3)
0.0606(11)
0.2758(18)
0.0441(8)
N13 0.6589(2)
-0.0247(10) 0.5690(18)
0.0441(8)
C14 0.6305(4)
0.0687(11)
0.5458(24)
0.0441(8)
C15 0.6167(4)
0.1017(12)
0.7617(21)
0.0441(8)
O16 0.6208(2)
0.0355(7)
0.9225(12)
0.0441(8)
O17 0.5932(2)
0.2022(8)
0.7932(13)
0.0441(8)
C18 0.5718(4)
0.2320(10)
0.9863(19)
0.0441(8)
C19 0.5335(4)
0.1724(11)
0.9769(18)
0.0441(8)
C20 0.5938(4)
0.0514(12)
0.4097(19)
0.0441(8)
C21 0.5655(3)
-0.0472(11) 0.4955(20)
0.0441(8)
C22 0.5341(4)
-0.0740(14) 0.3477(23)
0.0441(8)
C23 0.4996(5)
-0.0071(12) 0.3717(19)
0.0441(8)
C24 0.4677(4)
-0.0337(12) 0.1934(26)
0.0441(8)
C25 0.4794(4)
-0.1024(12) 0.0282(22)
0.0441(8)
C26 0.5113(5)
-0.1707(12) 0.0396(21)
0.0441(8)
C27 0.5418(4)
-0.1410(13) 0.1638(26)
0.0441(8)
C28 0.3701(5)
1.0488(11)
1.8396(18)
0.0542(14)
C29 0.3649(5)
1.1683(12)
1.9105(18)
0.0542(14)
O30 0.3682(3)
1.2621(6)
1.7808(12)
0.0542(14)
O31 0.3681(3)
1.2054(8)
2.0984(13)
0.0542(14)
C32 0.3643(5)
0.9489(11)
1.9198(20)
0.0542(14)
C33 0.3582(5)
0.9184(11)
2.1378(21)
0.0542(14)
O34 0.3525(3)
0.8142(7)
2.2096(13)
0.0542(14)
O35 0.3642(3)
1.0115(8)
2.2671(13)
0.0542(14)
a) Numbers in parentheses are statistical esd’s from the Rietveld program. It is widely
known that realistic error estimates from powder experiments are substantially larger than
these numbers, perhaps a factor of ten.
19
Table 4. Selective Torsion Angles of Enalapril
Torsion angle
C15-O17-C18-C19
C27-C22-C21-C20
C22-C21-C20-C14
C21-C20-C14-N13
C20-C14-N13-C11
φ1 (C9-C11-N13-C14)
ψ1 (N3-C9-C11-N13)
ω (C4-N3-C9-C11)
φ2 (C6-C4-N3-C9)
ψ2 (O8-C6-C4-N3)
Form I(º)
-166.7
83.9
179.4
67.7
57.7
175.0
156.3
-178
-52.8
139.7
Form II(º)
-87.5
72.9
-171.6
65.5
55.7
175.0
164.1
-179
-59.2
143.7
20
a
b
5
10
15
20
25
30
35
40
Cu Kα1 2θ
Figure 1
21
200
150
x10
6
a
100
***
** * *
* **
**
** *
** **
* **
**
** * *
50
b
****
** * *
0
300
250
200
150
100
50
0
Figure 2
22
x 10
5
10
15
20
25
30
35
40
45
2θ
Figure 3
23
Figure 4
24
a
b
Figure 5
25
100
a
95
T%
90
754.9
b
85
80
750.5
75
70
900
850
800
750
700
650
600
-1
wave number (cm )
Figure 6
26
140
120
a
x10
6
100
80
60
40
b
20
0
150
140
130
120
110
Figure 7
27
x10
6
150
a
100
b
50
0
40
30
20
10
0
Figure 8
a
28
b
Figure 9
29